The invention relates to the field of crystal oscillators, and relates in particular to the field of low power crystal oscillators.
There is an increasing need for a growing number of types of circuits to be designed to operate on battery power. Battery power and battery life, however, are typically limiting factors in the performance of a battery power circuit. Moreover, as battery power circuits become more complex they may require more power and/or performance life. There is a growing need, therefore, for crystal oscillators to require as little of the power as necessary, preserving much of the battery power for the remaining elements of the circuit.
There is also a continuous need for such circuits to be as small as possible and to have as few external connections as possible, given the increasing demand for including a growing number of circuit elements to be included in as little circuit and/or integrated circuit real estate as possible.
Further, as wireless communication systems (e.g., radio frequency telecommunication systems) have become increasingly popular, a demand has developed for less expensive yet efficient crystal oscillators. Such oscillators may be used with any of a plurality of standards for radio frequency modulation in wireless communication devices, such as global systems for mobile communication (GSM), which operate at 900 MHz. or digital cellular systems (DCS), which operate at 1800 MHz.
There is a need, therefore, for a low power crystal oscillator that requires a small amount of area on an integrated circuit and as few external connections on the integrated circuit as possible. It is further desirable to provide a low power crystal oscillator that requires only one external connection on an integrated circuit in addition to the source voltage and ground.
The invention provides a crystal oscillator circuit including a differential amplifier, a positive feedback assembly, and a series resonant crystal assembly. The differential amplifier includes a first transistor and a second transistor. The positive feedback assembly is coupled each of the first and second transistors, and has a loop gain of greater than unity. The series resonant crystal assembly is coupled to one of the first and second transistors, and includes a crystal and a capacitor.